This invention relates generally to a color video system having a color filter wheel for generating color data regarding the primary colors utilized in the system and, in particular, to an open loop drive unit for rotating and filter wheel in synchronization with the video field rate.
As evidenced by the disclosures in U.S. Pat. Nos. 2,921,118 and 3,654,385, the use of color filter wheels in video systems has been known for quite some time. For the most part, the drive units for turning the wheels have been rather complex and costly devices generally requiring the use of a feedback network to control the speed of the drive motor. In one such application, an accurately machined light chopper containing a relatively large number of equally spaced grooves or slits is operatively connected to the wheel. The light chopper is arranged to pass between a light source and a photodetector to provide information concerning the wheel's position and its speed. This type of equipment generally consumes a good deal of space and, because of the amount and complexity of the equipment involved, it is relatively expensive and sometimes difficult to maintain.
The use of color video system for viewing remote body regions has become a powerful diagnostic tool in the medical field. In U.S. Pat. No. 4,074,306, a color endoscope is disclosed wherein a filter wheel is employed to sequentially illuminate a target to provide color separated image information which is processed in the video system to present a color separated image of the target area one over the other upon a a television screen. As can be seen, both a sharp image and a faithful reproduction of the original colors is important in making an accurate diagnosis and any loss of image registration on the like will adversely affect the results of the examination.
It should also be noted that most color filter wheel drive systems now in use employ either synchronous AC motors or variable speed DC motors to drive the wheel. In either case, the efficiency of the motor and its associated drive circuitry is relatively low. The use of a stepping motor is therefore very attractive, however, controlling the motor at the required speeds has heretofore been a problem in the art.